1. Field of the Invention
This invention relates to a plain bearing, and more particularly to a plain bearing comprising a bearing metal layer having an uneven textured portion formed on the inner surface thereof, and an overlay layer provided on the inner surface of the bearing metal layer to cover the uneven textured potion.
2. Description of the Related Art
There is known a plain bearing comprising a back metal layer and a bearing metal layer bonded to the back metal layer, which is further provided with an overlay layer made of a soft material such as a Pbxe2x80x94Sn alloy on the surface of the bearing metal layer so as to enhance the conformability and the foreign matter embeddability of the bearing. Moreover, there is also known the above plain bearing further comprising an intermediate layer made from Ni or the like provided between the bearing metal layer and the overlay layer so as to prevent components of the overlay layer from diffusing into the bearing metal layer and improve the adhesiveness of the overlay layer.
In case of the bearings, when the overlay layer wears to expose the hard bearing metal layer or the harder intermediate layer over a wide area, an associated shaft slides directly on the hard layers over the wide area, so that the bearings readily cause the seizure. For the purpose of preventing this, there is also known a plain bearing having groove-like recesses 64 formed on the inner surface of the bearing metal layer 61 by boring in the axial direction, and the intermediate layer 62 and the overlay layer 63 provided on the recesses 64, as shown in FIGS. 7 and 8, so that even when the overlay layer 63 wears to the level M shown in FIG. 7, the soft overlay layer 63 provided in the recesses 64 still exists together with the intermediate layer 62 and the bearing metal layer 61 and form a mixing condition so as not to expose the intermediate layer 62 and the bearing metal layer 61 over a wide area, whereby maintaining the anti-seizure property.
In case of the bearing, if the overlay layer is exposed widely, the fatigue resistance and the wear resistance are deteriorated, while the conformability and the foreign matter embeddability are enhanced. On the contrary, if the bearing metal layer is exposed widely, the conformability and the foreign matter embeddability are deteriorated, namely the anti-seizure property is deteriorated, while the fatigue resistance and the wear resistance are enhanced.
Since the recesses of the conventional plain bearing have the same depth with each other, when the inner surface of the bearing wears to the bottoms of the recesses especially in a load carrying region, the hard bearing metal layer suddenly becomes exposed in a wide area, so that the bearing readily causes the seizure. Particularly in recent years, there are tendencies to avoid using lead in view of an environmental problem and to harden the bearing metal layer, and thus it has been worried that the seizure is readily caused.
Accordingly, it is an object of the present invention to enhance the anti-seizure property of a plain bearing by providing a plurality of recesses different in depth so as to prevent the bearing metal layer and the overlay layer from being exposed in a wide area suddenly.
According to the invention, there is provided a plain bearing comprising a back metal layer; a bearing metal layer provided on the inner surface of the back metal layer, which has an uneven textured portion formed on the inner surface thereof; and a soft overlay layer provided on the inner surface of the bearing metal layer to fill recesses of the uneven textured portion; wherein the recesses comprise at least two kinds of recesses different in depth.
According to the above feature, the overlay layer covering the uneven textured portion develops the conformability at the beginning of the use. Then, when the inner surface of the plain bearing wears to expose the bearing metal forming protrusions of the uneven textured portion, the overlay layer and the bearing metal layer appear and form a mixing condition. Further, even when those layers wear to the bottoms of the shallower recesses, the deeper recesses remain so that those layers still appear and form a mixing condition. As a result, the bearing metal layer is not exposed over a wide area in a load carrying region, and thus the preferred condition hardly to cause the seizure is maintained.
Typically, the bearing metal layer is made from a Cu or an Al alloy. The overlay layer material may be optionally selected from the group of Pb, a Pb alloy, Sn, a Sn alloy, and a resin suitable for a sliding-contact application, e.g. PTFE and PFA.
Moreover, although the overlay layer may be directly provided on the bearing metal layer, it also may be possible to provide an intermediate layer between the bearing metal layer and the overlay layer so as to enhance the adhesiveness and prevent components of the overlay layer from diffusing into the bearing metal layer. The intermediate layer material may be optionally selected from the group of Ni, Ag, a Ni alloy, an Ag alloy, Cu, a Cu alloy, Co, a Co alloy, Sn, a Sn alloy, Zn and a Zn alloy, for example.
Typically, the overlay layer is made from a lead-tin alloy. However, the overlay layer may be made from another soft material, which does not include lead, such as a resin or a tin base alloy.
According to the invention, it is possible to provide a plain bearing excellent in the anti-seizure property.
With reference to the attached drawings, preferred embodiments of the plain bearing according to the invention will be described herein below.